Re: [Harp-L] Flat sanding problems

[David Payne <dave@xxxxxxxxxxxxxxxxxxxxxx>]

Be it known to all and sundry when buying a custom harmonica, or custom comb, having it brought down to whatever oddball thickness you've got in your head is not an easy request to fulfill. people think it is, but it ain't.

Ok, Boris, look. Here's the deal. It's pretty much impossible to get anything perfectly flat by hand... you just make it flat as possible and if you're just flat sanding for flatness, you don't remove that much material and everything is pretty flat, but like I said earlier, when you flatsand more strokes than you should be, all these imperfections are magnified and you get a comb like you have... a comb that sucks. 
So... You know all that by now. The most reasonable solution for you is follow this rule:

The Payne Law of the Balance of Imperfection.

which says, imperfection can be properly used to create perfection. 

So, you can take a little material off and make something flat... BUT you can't take a lot off and make it flat, nor can you flatsand this big lump in the middle. With flatsanding, that lump will never shrink, it will only grow. What I would do, in this situation, is remove the lump by hand... Maybe a very fine rasp (one that won't catch and break comb teeth, you know, and use it in a downward motion down the comb teeth). I actually have tiny little planing tools for stuff like this. This is something you'll want to use a pair of calipers to measure thickness as you remove material. You are not trying to get it flat now. You are just trying to get it flat enough so that normal flatsanding will be capable of making it flat. 

There is another problem. I think you would have said something if you had noticed it. I'm guessing you haven't. Your comb is, if you look at it from the side, pyramid shaped. When you sand a comb, you are always removing more material from the comb teeth than from parts where the comb is more solid. This means the back of the harp on the high end is thicker than the low and the entire back is thicker than the front. Whenever I've thinned a comb, I've compensated by applying more pressure to the back high corner, getting it a little too low, then sanding it normally to make it flat. 
Why does more material come off the comb teeth? Because the leading edge of the comb teeth will remove material - and the farther any part of the comb goes along the sanding surface without a leading edge to clean sanded material away, the less material is removed. That sanded away material actually acts as a lubricant. You can see it build up along the teeth as you sand. Other parts of the comb are lubricated by the sanded material.

There is a sanding cloth that looks like a screen. It gives you a little leeway, because the material can go into those holes in the screen. But you have to clean it all every few strokes, or move to a clean part of the sanding surface - but you still have to clean when you run out of clean places on the surface. That would be the easiest method I think to thin a comb.  

 
David Payne
www.elkriverharmonicas.com
www.hetrickharmonica.com


________________________________
 From: Vern <jevern@xxxxxxx>
To: Boris Plotnikov <ploboris@xxxxxxxxx> 
Cc: harp-l@xxxxxxxxxx 
Sent: Thursday, December 27, 2012 6:41 PM
Subject: Re: [Harp-L] Flat sanding problems
 
By hand-sanding away .5 mm of thickness, you will inevitably spoil the flatness. 

I suggest the following:

Method 1.  Routing
Chuck a 1/4" or larger, new, sharp router bit or end mill in your drill press.  
Use maximum rpm.
Clamp the comb level in the drill press vise.
Set the bit height to remove .5 mm.
Slowly and gently push the vise around to remove the unwanted comb thickness.
Push the vise so that the blade tips of the bit move tangent to each partition and toward the thick back of the comb. 
Keep your hands away from that router bit!

"But I don't have a drill press!" you say....

...If the combs have any value, then the ones that you avoid ruining should quickly pay for a small drill press that you can also use for other purposes.  The satisfaction of doing it right has value.


Method 2. Guided Sanding
Attach the comb to the flat surface with double-sticky carpet tape.  Then glue coarse sandpaper to an "upside-down" sanding block that is both 6 inches longer and wider than the comb.  Put four (brass to avoid scratching the reference flat) screws near the corners of the block to act as legs. Adjust them to protrude downward by the desired 5.5 mm comb thickness.  A 12" x 12" granite floor tile makes a good, easily-obtained and inexpensive reference flat.  

Then have at it.  Any part of the comb that is thicker than the leg-length gets sanded away and any part thinner than the leg-length does not. Use wet sandpaper for plastic combs.

If you find a better method, I'd like to know about it. 

Vern

On Dec 27, 2012, at 1:24 PM, Boris Plotnikov wrote:

> Thanks guys for responce. Just for experiment I just tried to flat sand
> wood 1847 comb. The same crap, borders is thinner than center. I'll buy
> coarser sandpaper tomorrow and will try again.